Large valves are often designed so that the fluid in the line in which the valve is placed is used to assist in closing the valve and maintaining it closed. For example, in large steam isolation valves the main poppet is usually integral with a closure member which includes a piston which is partially placed in an auxiliary chamber which is fed with fluid from the line so that the poppet can be moved in a direction generally away from the chamber and toward the valve seat without having to exert a large force on the closure member through the valve stem. The fluid directed continuously into the chamber as the valve is closing so that there is no large pressure differential across the closure member.
There are instances where it is desirable to add to the force which acts on the closure member to maintain the valve shut. For example, the valve will periodically be tested for seat tightness. Such tests are usually conducted by applying a pneumatic pressure, which is considerably less than the operating line pressure upstream and downstream of the valve. A pneumatic test can easily apply a pressure which unseats the valve and therefore prevent a valid evaluation of the seat tightness.